1. Field of the Invention
This invention relates generally to the field of seed layers for subsequent metallization. In particular, this invention relates to methods for repairing seed layers prior to metallization.
2. Description of the Related Art
The trend toward smaller microelectronic devices, such as those with sub-micron geometries, has resulted in devices with multiple metallization layers to handle the higher densities. One common metal used for forming metal lines, also referred to as wiring, on a semiconductor wafer is aluminum. Aluminum has the advantage of being relatively inexpensive, having low resistivity, and being relatively easy to etch. Aluminum has also been used to form interconnections in vias to connect the different metal layers. However, as the size of via/contact holes shrinks to the sub-micron region, a step coverage problem appears which in turn can cause reliability problems when using aluminum to form the interconnections between the different metal layers. Such poor step coverage results in high current density and enhances electromigration.
One approach to providing improved interconnection paths in the vias is to form completely filled plugs by using metals such as tungsten while using aluminum for the metal layers. However, tungsten processes are expensive and complicated, tungsten has high resistivity, and tungsten plugs are susceptible to voids and form poor interfaces with the wiring layers.
Copper has been proposed as a replacement material for interconnect metallizations. Copper has the advantages of improved electrical properties as compared to tungsten and better electromigration property and lower resistivity than aluminum. The drawbacks to copper are that it is more difficult to etch as compared to aluminum and tungsten and it has a tendency to migrate into the dielectric layer, such as silicon dioxide. To prevent such migration, a barrier layer, such as titanium nitride, tantalum nitride and the like, must be used prior to the depositing of a copper layer.
Typical techniques for applying a copper layer, such as electrochemical deposition, are only suitable for applying copper to an electrically conductive layer. Thus, an underlying conductive seed layer, typically a metal seed layer such as copper, is generally applied to the substrate prior to electrochemically depositing copper. Such seed layers may be applied by a variety of methods, such as physical vapor deposition (xe2x80x9cPVDxe2x80x9d) and chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). Typically, seed layers are thin in comparison to other metal layers, such as from 50 to 1500 angstroms thick. Such metal seed layers, particularly copper seed layers, may suffer from problems such as metal oxide both on the surface of the seed layer and in the bulk of the layer as well as discontinuities in the layer.
Oxide on a metal seed layer, particularly a copper seed layer, interferes with subsequent copper deposition. Such oxide forms from exposure of the metal seed layer to oxygen, such as air. The longer such seed layer is exposed to oxygen, the greater the amount of oxide formation. Where a copper seed layer is thin, the copper oxide may exist as copper oxide throughout the layer. In other areas of electroplating, such as in electronics finishing, copper oxide layers are typically removed by acidic etching baths. These baths dissolve the oxide layer, leaving a copper metal surface. Such etching processes are not generally applicable to copper seed layers because of the thinness of the seed layer. As the oxide is removed from the seed layer surface there is the danger that the entire seed layer may be removed in places, creating discontinuities in the seed layer.
U.S. Pat. No. 5,824,599 (Schacham-Diamand et al.) discloses a method of preventing oxide formation on the surface of a copper seed layer by conformally blanket depositing under vacuum a catalytic copper layer over a barrier layer on a wafer and then, without breaking the vacuum, depositing a protective aluminum layer over the catalytic copper layer. The wafer is then subjected to an electroless copper deposition solution which removes the protective aluminum layer exposing the underlying catalytic copper layer and then electrolessly deposits copper thereon. However, such method requires the use of a second metal, aluminum, which adds to the cost of the process and the presence of any unremoved protective layer prior to the electroless deposition of the copper may cause problems in the final product, such as an increase in resistivity. In addition, the dissolved aluminum may build up in the electroless copper bath, which could also cause problems in the final product.
Discontinuities or voids are areas in the seed layer where coverage of the metal, such as copper, is incomplete or lacking. Such discontinuities can arise from insufficient blanket deposition of the metal layer, such as depositing the metal in a line of sight fashion. In order for a complete metal layer to be electrochemically deposited on such a seed layer, the discontinuities must be filled in prior to or during the deposition of the final metal layer, or else voids in the final metal layer may occur.
PCT patent application number WO 99/47731 (Chen) discloses a method of providing a seed layer by first vapor depositing an ultra-thin seed layer followed by electrochemically enhancing the ultra-thin seed layer to form a final seed layer. According to this patent application, such a two step process provides a seed layer having reduced discontinuities. The copper seed layer is enhanced by using an alkaline electrolytic bath. Acid electrolytic baths for the seed layer enhancement are disclosed to be problematic due to the fact that voids in the seed layer can be created and thus providing poor uniformity in the metal layer deposited thereon. One using this method to enhance a seed layer would have to rinse and neutralize the seed layer before using conventional acidic electrolytic plating baths. In addition, a manufacturer using such alkaline enhancement method in combination with an acid electroplating bath would have to double the number of plating heads on the plating tool or throughput would decrease.
Thus, there is a continuing need for methods of repairing seed layers that remove any oxide surface formed, that do not require the use of additional metals, that enhance the lateral growth of seed layers to reduce or remove discontinuities, and that are compatible with commercial metal deposition processes.
The inventors have surprisingly found that a metal oxide layer may be easily removed from a seed layer surface without the use of an etchant solution and without increasing the size and number of seed layer discontinuities. The inventors have further found a method for enhancing the lateral growth of a seed layer using an acidic electrolytic bath.
In one aspect, the present invention is directed to a method of providing a metal seed layer substantially free of metal oxide disposed on a substrate including the steps of contacting a metal seed layer disposed on a substrate with an aqueous solution having a pH maintained in the range of about 6.5 to about 13 and subjecting the solution to a voltage of from about 0.1 to 5 volts.
In a second aspect, the present invention is directed to a method of providing a metal seed layer substantially free of discontinuities disposed on a substrate comprising the steps of contacting a metal seed layer disposed on a substrate with an acidic electrolyte bath and subjecting the bath to a current density in the range of up to about 0.1 A/cm2, wherein the acidic electrolyte bath comprises one or more acids, one or more copper compounds, one or more suppressors and water, wherein the acidic electrolyte bath is free of accelerators.
In a third aspect, the present invention provides a method of providing a metal seed layer substantially free of metal oxide and substantially free of discontinuities disposed on a substrate including the steps of contacting a metal seed layer disposed on a substrate with an aqueous solution having a pH maintained in the range of about 6.5 to about 13, subjecting the aqueous solution to a voltage of from about 0.1 to 5 volts, removing the substrate, optionally rinsing the metal seed layer disposed on the substrate, contacting the metal seed layer with an acidic electrolyte bath and subjecting the acidic electrolyte bath to a current density in the range of up to about 0.1 A/cm2, wherein the acidic electrolyte bath includes one or more acids, one or more copper compounds, one or more suppressors and water, and wherein the acidic electrolyte bath is substantially free of accelerators.
In a fourth aspect, the present invention provides a plating bath composition for substantially repairing discontinuities in a metal seed layer disposed on a substrate including one or more acids, one or more copper compounds, one or more suppressors and water, wherein the bath is substantially free of accelerators.
In a fifth aspect, the present invention provides a method for providing a metallized layer disposed on a substrate including the steps of: a) optionally coating the substrate with a barrier layer; b) providing a metal seed layer on the surface of the substrate; c) repairing the metal seed layer by substantially repairing discontinuities in the metal seed layer including the step of contacting the metal seed layer with an acidic electrolyte bath and subjecting the bath to a current density in the range of up to about 0.1 A/cm2, wherein the acidic electrolyte bath includes one or more acids, one or more copper compounds, one or more suppressors and water, and wherein the acidic electrolyte bath is substantially free of accelerators; d) subjecting the repaired metal seed layer to a metal plating bath to provide a metallized layer disposed on the substrate; and e) optionally rinsing the metallized layer disposed on the substrate.
In a sixth aspect, the present invention provides a substrate having a metallized layer disposed thereon according to the method described above.